This invention relates to a method for making optical waveguides, and more particularly to a method for substantially continuously drying, consolidating and drawing an optical waveguide preform having a support member formed as an integral part of the preform body.
Optical waveguides, which are the most promising medium for transmission of high frequencies, normally consist of an optical filament having a transparent core surrounded by a transparent cladding having a refractive index which is lower than that of the core. A very thorough and complete discussion concerning operational theories of optical waveguides is contained in U.S. Pat. No. 3,157,726 issued to Hicks et al., and in the publication "Cylindrical Dielectric Waveguide Modes" by E. Snitzer, Journal of the Optical Society of America, Vol. 51, No. 5, pp. 491-498, May 1961. Other excellent sources of information concerning optical waveguides are "Fiber Optics--Principles and Applications" by N. S. Kapany, Academic Press, 1967, as well as "Fundamentals of Optical Fiber Communications" edited by Michael K. Barnoski, Academic Press, 1976.
Generally optical waveguides are made by either outside or inside vapor phase oxidation as hereinafter more particularly defined. The present invention is particularly suitable to the formation of optical waveguide preforms by the outside vapor phase oxidation process where a starting rod or tube is used as a starting member, bait, or mandrel, on the outside surface of which is deposited one or more layers of suitable material. Ordinarily, the starting member or mandrel is removed leaving the deposited structure as a preform for the subsequent waveguide. Where a tube is employed as a starting member and is either left in place or removed, a center hole exists in the same manner as if a solid rod was removed from the deposited material. The center hole must thereafter be closed before or during drawing and a solid elongated waveguide filament drawn. In the outside process, either the starting rod forms the core of the ultimate waveguide or the first deposited layers form the core of the ultimate waveguide, while the latter deposited layers form the cladding thereof.
When a starting member or mandrel is used and is intended to remain with the structure to ultimately form the core of the waveguide, substantial exterior surface preparation such as cleaning, smoothing, or the like is necessary before deposition thereto may take place. In addition, if the starting member or mandrel is intended to remain and become the core of the waveguide, it must be a very high purity glass, and have optical properties equivalent to the quality of the waveguide core, for example, low signal attenuation and equivalent index of refraction. However, when the starting member or mandrel is removed, other problems are presented. Among these problems are that the preform is free standing such that handling and further processing of the preform is made more difficult. For example, when the starting member or mandrel is removed the resulting preform is a porous structure having a central aperture. Such a porous structure must thereafter be consolidated into a solid structure and the central aperture must be closed either by drawing or other preliminary processing well known in the art. Handling of such a porous structure during consolidation and subsequent drawing and/or hole closing requires special preparation and handling. One manner for preparing the porous preform for consolidation is to drill a hole through the structure perpendicular to the longitudinal axis thereof near one end of the structure to permit a platinum wire, or the like, to be passed through the drilled hole for hanging the preform vertically during consolidation. Unfortunately, however, the porous preform is structurally vulnerable during such drilling operation and frequently leads to the destruction of the preform due to lengthwise splits or cracks and always allows liberated soot from the drilling to contaminate the center hole of the preform.